Gas turbine engines, such as those used to power aircraft, include an array of fan blades positioned near the forward end of the engine. The blades project from a hub capable of rotation about an engine axis. A case circumscribes the tips of the blades to form a duct that defines the radially outer boundary of a working medium fluid flowpath. The duct internal wall often includes an acoustic liner residing axially forwardly of the blades for attenuating noise generated by the engine. The acoustic liner is usually made of several circumferentially extending segments whose edges are joined to each other by circumferentially distributed splices. The splices do not necessarily possess the noise attenuating properties present in the rest of the acoustic liner. Stated more technically, the acoustic impedance of the splices differs from the acoustic impedance of the rest of the acoustic liner.
During engine operation, the linear speed of each blade (sometimes referred to as wheel speed) increases with increasing radius. As a result, the radially outer portions of the blades can operate in a transonic or supersonic regime. The transonic regime is characterized by the formation, forward of the blades, of localized aerodynamic shocks that do not propagate upstream. The supersonic regime is characterized by shocks that propagate upstream. In both flow regimes the splices joining the acoustic liner segments can cause the acoustic energy of the shocks to be scattered into acoustic waves that may include modes whose pressure fluctuations are not necessarily radially close to the acoustic liner. Accordingly, the acoustic liner may be ineffective at attenuating these modes. The associated noise can propagate forwardly out of the duct.
In principle, the shock associated noise may be attenuated by using a single piece liner, or by using splices that possess noise attenuating properties similar to those of the acoustic liner. However these approaches are not always practical or cost effective.
Turbine engine ducts may also employ treatments, usually referred to as casing treatments, for enhancing the aerodynamic stability of the blades. One type of casing treatment includes circumferentially extending grooves in the case axially aligned with the blades. The grooves are axially separated from each other by intergroove rails. The grooves are interrupted by axially extending, circumferentially distributed partitions. The acoustic impedance of the partitions differs from the acoustic impedance of the grooves so that, like the acoustic splices described above, the partitions can cause shock related noise to propagate forwardly out of the duct. Since the partitions are desirable for aerodynamic reasons, they usually cannot be eliminated. Moreover, they cannot be made acoustically similar to the grooves.
Thus, it is seen that certain features in a duct, such as acoustic liners and casing treatment partitions, can cause shock related noise to propagate out of the duct. What is needed, therefore, is a way to include such features, without causing undesirable acoustic behavior.